Currently, as wireless communication booms, signals in the radio interface tend to suffer from various interferences. Particularly, global positioning system (GPS) applications potentially will experience a mixture of both narrowband and wideband interferences. The nominal power of a signal at an antenna port of a GPS receiver is about −130 dBm, while the thermal noise level is about −110 dBm. Therefore, in normal operation, the received GPS signal is buried under the noise floor.
FIG. 1 is a block diagram of a prior art GPS receiver 100. Typically, a mixture of the GPS signal and the thermal noise is firstly converted to an intermediate frequency (IF) signal through a conventional RF filter, low noise amplifier and down-converting mixer. Then, after a complex filtering process, the IF signal is further amplified by a variable gain amplifier (VGA) 110 and converted from an analog format to a digital format by a 2-bit analog to digital converter (ADC) 120. The amplified IF signal should have a voltage level that satisfies the dynamic range requirement of the ADC 120. In order to control the voltage level of the amplified IF signal, an automatic gain control (AGC) loop 130 with a capacitor 140 is designed for regulating the gain of the VGA 110. The VGA 110, the ADC 120, the AGC loop 130 and the capacitor 140 form a signal amplification and digitization circuit.
FIG. 2 is a schematic diagram of the signal amplification and digitization circuit in FIG. 1. After being amplified by the VGA 110 according to a predetermined gain, the IF signal is then converted to a digital magnitude signal MGNA and to a digital sign signal SIGN by the 2-bit ADC 120. The 2-bit ADC 120 includes a current source 121 and a current sink 123. When the output from the VGA 110 is either larger than a positive reference signal Vref or smaller than a negative reference signal −Vref, the current sink 123 will sink a current lout from the capacitor 140. Otherwise, the current source 121 will source the current lout into the capacitor 140. At steady state condition, a DC voltage at the capacitor 140 is constant and fed back to the VGA 110. The feedback loop is usually called the AGC loop and used to regulate the predetermined gain. Generally, a time constant of the AGC loop has to be in the order of millisecond (ms), and therefore the capacitance of the capacitor 140 has to be in the order of nanofarads (nF). To have such a large capacitance, the capacitor 140 has to be realized off-chip as a discrete and external component and thus increases the overall cost of the circuitry.
After the aforementioned process, though the thermal noise still exists, a base-band correlator 150 in FIG. 1 can obtain a proper post-correlation signal-to-noise ratio (SNR) by correlating the digital signals MAGN and SIGN for a long period. However, for constant envelope continuous-wave (CW) interference, the SNR degradation is much greater than the thermal noise and the GPS receiver must reduce the SNR degradation prior to the correlation process. Interference is generally mitigated at the ADC 120. Furthermore, the CW interference has much larger power than the thermal noise, and therefore the AGC loop 130 should ensure that the gain of the VGA varies over a dynamic range in order to maintain an optimal signal amplitude at the input of the ADC 120.
It is thus desirous to have an ADC with interference rejection capability that is capable of implementing the aforementioned AGC loop directly so that a large external capacitor is not required. It is to such an ADC and AGC method thereof that the present invention is primarily directed.